The invention relates to a technique for detecting that head driver ICs of an ink jet printer have reached a predetermined temperature or higher.
FIG. 1 shows the outline of hardware of an ink jet printer constituted by piezoelectric vibrators serving as elements for ejecting ink from nozzles. As shown in this figure, a controller 102 is a control board which is to be implemented in a printer and causes a print engine 103 to perform printing operation complying with data entered by way of an interface 104. A CPU (central processing unit) 123 executes a program stored in a ROM (read-only memory) 121, thereby controlling individual sections provided in the controller 102. A main bus of the controller 102 is connected to a RAM (random access memory) 122 serving as a primary storage device of the CPU 123 and to a PROM 124 (programmable read-only memory) for recording various types of setting items.
An IC chip 126 provided in the controller 102 actually sends various types of signals to the print engine 103 pursuant to a print instruction construed by the CPU 123. The custom IC chip 126 serves as an engine controller which performs a centralized control operation pertaining to printing and driving of the printer.
First, a signal to be sent from the custom IC chip 126 is a drive signal for controlling a motor. The motor drive signal sent by way of a signal line 128 effects feeding of print paper or actuation of a head unit 131 mounted on an unillustrated carriage.
Signals used directly for printing operation are a digital signal and an analog signal for driving a switcher 135. A digital signal, representing whether ink is ejected from each nozzle, is delivered without modification via a signal line 132 to a switcher 135. A signal to be used for determining the size of ink droplets is temporarily sent to a digital/analog converter 133 in the form of a set of digital vector data through via a signal line 134. The digital signal is converted into an analog trapezoidal waveform, an example of which is shown in FIG. 2, and then delivered to the switcher 135.
In an ink jet printer which causes nozzles to eject ink, by utilization of expanding action of a piezoelectric vibrator 137 shown in FIG. 1, the switcher 135 assumes a configuration such as that shown in FIG. 3.
In FIG. 3, each of the switchers 135 (135-1 through 135-n) has two input ports; that is, a digital input port and an analog input port. An analog drive signal complying with these inputs is output to piezoelectric vibrators 137 (137-1 through 137-n) by way of signal lines 136 (136-1 through 136-n). The digital signal line 132 is connected to an input terminal of the custom IC chip 126 (FIG. 1). Data which have been serially input by way of the custom IC chip 126 are serially transferred to each of shift registers 141 (141-1 through 141-n) in accordance with a clock signal CLK, and latched in each of latches 142 (142-1 through 142-n) at a predetermined timing defined by a latch signal LAT. The thus-latched data are output to the switcher 135.
The switcher 135 outputs to the piezoelectric vibrator 137 an amplitude represented by the analog waveform signal (FIG. 2) at a timing defined by the digital signal. By means of provision of such a switcher for each nozzle, ink droplets of arbitrary size can be ejected at an arbitrary timing. Such switchers equal in number to nozzles are integrated, thereby constituting a single switching semiconductor element.
In a standard assembly process for an ink jet printer, one head is constructed of a total of eight groups of nozzles; that is, a group of black ink nozzles, a group of yellow ink nozzles, a group of cyan ink nozzles, a group of magenta ink nozzles, a group of light cyan ink nozzles, a group of light magenta ink nozzles, and a group of dark yellow ink nozzles. A switching semiconductor element is provided for each of the nozzle groups.
The thus-integrated switcher has a heat resisting temperature at which normal operation of the switcher is guaranteed. Similarly, a heat-resisting temperature is determined also for a conductive adhesive or the like to be used for assembling constituent components of the head. Hence, in order to prevent a hindrance to normal operation of individual constituent components or thermal breakdown of the components, which would otherwise be caused by idle ejecting operation stemming from depletion of ink, a diode which is to serve as a semiconductor element for detecting a temperature is incorporated in each of the ink nozzle drive switching elements. An internal temperature of the semiconductor element is measured by a voltage output from the diode.
FIG. 4 shows a configuration for measuring a temperature, by potential differences among four diodes connected between a constant current source CS and ground. The diodes have physical properties whose output voltages are determined in accordance with a temperature environment when constant power is supplied from the constant current source CS.
An output from the temperature detecting semiconductor element (diode) having such a configuration is returned to the previously-described custom IC chip 26 from the carriage having the printer head unit 131 mounted thereon, by way of a signal line of a flexible flat cable (i.e., the signal line 127 shown in FIG. 1). By utilization of a value of the output, the custom IC chip 26 performs various types of print control operations, such as suspension of a printing operation in the event of generation of, e.g., overheat.
In actual assembly processes relating to manufacture of a printer, a temperature detecting diode has already been incorporated into an ink nozzle drive switching semiconductor element supplied as a component. When the ink nozzle drive switching semiconductor element is produced by way of a single manufacturing process, errors resulting from variations in quality may arise. However, the ink nozzle drive switching semiconductor elements do not vary from each other in terms of principal characteristics; that is, the quantity of heat stemming from switching actions or a characteristic of a voltage changing in accordance with the temperature of a diode.
From the viewpoint of the quantity of supplied parts and costs incurred in material procurement, in many cases parts produced through different manufacturing processes are used in a single printer at a site for controlling manufacturing processes. Even in the case of an ink nozzle drive switching semiconductor element, semiconductor elements produced through different manufacturing processes are employed. In such a case, semiconductor elements supplied from certain manufacturing processes often differ from those supplied from other manufacturing processes in terms of characteristics of diodes built in the semiconductor elements.
FIG. 5 is a graph representing the relationship between characteristics of diodes. In the graph, the vertical axis represents a voltage of an anode output 50 shown in FIG. 4. In other words, the vertical axis corresponds to a total potential difference between the anodes and cathodes of four diodes connected in series with each other. The horizontal axis represents temperatures of locations where a temperature detecting circuit, such as that shown in FIG. 4, is disposed.
The graph shows a physical property of a diode built in a switcher produced through production processes A and that of a diode built in a switcher produced through production processes B, wherein output voltages of the diodes are determined in accordance with a temperature environment. Specifically, a diode produced through manufacturing processes A produces an anode output of 2.4 V in a temperature environment of 25° C., whilst a diode produced through manufacturing processes B produces an anode output of 2.0 V in a temperature environment of 25° C.
Further, the graph also shows a characteristic of a rate at which an output voltage is changed in accordance with changes in a temperature environment; that is, different gradients of respective line segments of the graph.
Variations exist in respective diodes produced through the manufacturing processes A and in those produced through the manufacturing processes B, the variations being attributable to individual differences. In the graph, standard values of products are denoted by solid lines, and the range of variation is denoted by dashed lines.
For instance, in a case where the guaranteed heat-proof temperature of the switcher is 120° C., the switcher is determined to be overheated when the anode output voltage has dropped to 1.3 V (i.e., the maximum value of the individual differences) in light of the temperature-voltage characteristic of the diode produced through the manufacturing processes B. In a case where the temperature of the printer is controlled on the assumption of a characteristic of a rate at which the output voltage of the diode changes, the anode output voltage is considered to have dropped to 2.1 V (i.e., the maximum value of the individual differences) when the ink nozzle driver switching semiconductor element produced through the manufacturing processes A is used for a product. Accordingly, overheat of the switcher cannot be detected.
As mentioned previously, a related-art head driver IC temperature detector of an ink jet printer measures anode voltages of diodes provided in a head driver IC, and the temperatures of junctions of transistors provided in the IC are detected by temperature characteristics of the anode voltages.
However, since diodes have great variations in characteristics thereof, a result of mere measurement of junction temperatures performed by the temperature detecting diodes provided in the head driver IC also includes a great variation.
In short, the above-described temperature detecting method encounters difficulty in detecting temperatures accurately, because of individual differences in anode voltage at a certain temperature or individual differences in temperature coefficient of an anode voltage.
By the way, the head driver ICs generate heat when they are driven, and the heat is dissipated by the ejected ink droplets. However, as a result of uninterrupted operation under extremely high toad, heat dissipation capacity may become insufficient. Moreover, in a state in which ink is not properly squirted for reasons of depletion of ink or clogging of nozzles, sufficient heat dissipation is not achieved. If printing operation is continued in such a state, the temperatures of respective head driver ICs rise further, potentially resulting in thermal destruction of the respective head driver ICs.
Therefore, in a related-art ink jet printer, attention is paid to the anode voltage of a diode provided in each of the head driver ICs changing in accordance with the ambient temperature. As shown in FIG. 6, the anode voltages of the diodes provided in four head driver ICs 1a, 1b, 1c, and 1d are output to a controller 4 which is provided in a printer main unit 3 and is constituted of, e.g., an ASIC, by way of respective signal lines 2a, 2b, 2c, and 2d provided in a flexible flat cable (FFC).
In the controller 4, the anode voltages are converted into digital values by an analog-to-digital converter 5, thereby detecting anode voltages of diodes of the respective head driver ICs. In accordance with the anode voltages, the temperatures of the respective head driver ICs 1a, 1b, 1c, and 1d are detected. When any one of the head driver ICs 1a, 1b, 1c, and 1d has reached a predetermined temperature or more, the controller 4 temporarily stops a printing operation, thereby lowering the temperatures of the head driver ICs 1a, 1b, 1c, and 1d. 
However, according to such a method of detecting the temperatures of the head driver ICs 1a, 1b, 1c, and 1d, analog signals pass through the signal lines 2a, 2b, 2c, and 2d provided in the comparatively long FFC 7 extending from the printer head 6 to the printer main unit 3. The analog signals are susceptible to the influence of noise, thereby deteriorating the accuracy of detection.
The anode voltages of the respective head drivers ICs 1a, 1b, 1c, and 1d are converted into digital signals by the analog-to-digital converter 5 within the controller 4, thereby prolonging a detection time and requiring provision of the analog-to-digital converter 5 within the controller 4. Accordingly, the controller 4 constituted of, e.g., an ASIC, becomes bulky.
Moreover, the lines 2a, 2b, 2c, and 2d must be provided in the FFC 7 in equal number with the head drier ICs. Further, the number of input pins of the controller 4 increases, thereby resulting in a cost hike.
In a case where a rupture has arisen in any one of the signal lines, a rise in the temperature of a corresponding head driver IC cannot be detected. Hence, the rise in the temperature of that head driver IC may be left undetected. Accordingly, damage may be inflicted on the printer head.
In this way, when a temperature detecting circuit has become broken as a result of occurrence of a rupture in a signal line or a short-circuit in any one of circuits for detecting temperatures, the configuration shown in FIG. 6 encounters difficulty in immediately detecting the failure and taking a countermeasure, such as suspension of operation of a printer.